Double-wall heat exchanger

ABSTRACT

An improved double-wall heat exchanger in which the safety “leak” space between heat exchange surfaces forming channels for the fluids exchanging heat is filled with a heat transfer medium with improved thermal conductivity in comparison to air. Pressure responsive rupture points allow release of the heat transfer medium and the heat exchange fluid in the event of failure of one of the containment surfaces. Metal-to-metal heat transfer points are dispersed in the gap to further enhance heat transfer between the hot and cold heat exchange fluid streams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/393,404, filed Sep. 12, 2016, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[NOT APPLICABLE]

FIELD OF THE INVENTION

The present invention relates generally to heat exchangers used in heating, ventilation, and air conditioning systems and, more particularly but without limitation, to double-wall heat exchangers.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with this description, serve to explain the principles of the invention. The drawings merely illustrate preferred embodiments of the invention and are not to be construed as limiting the scope of the invention.

FIG. 1 is a schematic diagram of a section of a double-wall plate heat exchanger made in accordance with a preferred embodiment of the present invention.

FIG. 2 is schematic diagram of a section of a double-wall tube-in-tube heat exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Sustainability initiatives and regulations in the heating, ventilation, air conditioning, and refrigeration (HVAC&R) industry drive an increase in the system efficiency and heat exchanger effectiveness thresholds. At the same time, safety standards and building codes require enhanced rigidity and durability in the heat exchanger designs providing an improved protection to the end user. These trends are contradictory and steer the heat exchanger design to one end of the spectrum or the other. Consequently, such competing objectives have led the industry to design larger heat exchangers that in turn increase the overall footprint and cost of the HVAC&R system.

Double-wall heat exchangers are an example of this ongoing struggle. Double-wall designs for brazed plate and tube-in-tube heat exchangers include a space or gap between the cooling and heating media. This gap is designed to prevent direct leakage from the high pressure side to a low pressure side, which in turn leads to the system over-pressurization and fluid cross-contamination. While the double-wall construction improves safety, the space between the heat exchange fluids significantly increases thermal resistance thereby reducing the effectiveness of the heat exchanger.

The present invention provides a double-wall heat exchanger that accommodates the goals of safety and improved efficiency. In accordance with the invention, the space between the double-walls is filled with thermally conductive heat transfer media. Additionally, intentional weak points or rupture points in the walls forming the boundaries of the gap may be designed to rupture and release pressure in the event of leakage to prevent over-pressurization. Still further, heat transfer may be enhanced by increasing a number of solid thermally conductive contact points between the surfaces forming the gap.

The present invention is applicable to any double-wall heat exchanger design, including without limitation brazed plate heat exchangers and tube-in-tube heat exchangers. As the specific designs of the various types of heat exchanger are well known, the entire heat exchangers will not be shown or described in detail herein. By way of example only, one double-wall brazed plate heat exchanger is shown and described in U.S. Pat. No. 9,163,882, entitled “Plate Heat Exchanger with Channels for ‘Leaking Fluid’” issued Oct. 20, 2015, and is incorporated herein by reference.

Turning now to the drawings and to FIG. 1 in particular, there is shown therein a diagrammatic illustration of a section of a plate heat exchanger and designated generally at 100. The heat exchanger comprises plates “P” with grooves or ribs that are aligned to form flow passages for heat exchange fluids. Typically, but not always, the flow passages direct a first heat exchange fluid in one direction and a second heat exchange fluid in an opposite direct. It will be understood that assembled heat exchanger comprises multiple plates that form multiple flow paths in each direction, typically in an alternating manner. As used herein, “first conduit” refers to the collective flow paths in the first direction, and “second conduit” refers to the collective flow paths in the second, opposite direction.

By way of example, as illustrated in FIG. 1, the plates P1 and P2 form flow passage A, Plates P3 and P4 form flow passage B, plates P5 and P6 form flow passage C, and plates P7 and P8 form flow passage D. As indicated by the arrows, flow passages A and C direct fluid in the first direction (upward as viewed in the drawings), and flow passages B and D direct fluid in the opposite direction (downward as viewed in the drawing). Thus, the flow passages A and C collectively comprise the first conduit for a first heat exchange fluid, and flow passages B and D collectively comprise the second conduit for the second heat exchange fluid.

With continued reference to FIG. 1 and as previously indicated, the heat exchanger 100 is designed as a double-wall heat exchanger. To that end, the plates P1-P8 are configured to form a gap or space between opposing flow passages, which space is referred to herein as a heat transfer space. More specifically, the external surfaces of the plates, that is, the surfaces that are outside of the flow passages and that oppose each other define the heat transfer space. Thus, the heat transfer space S1 is formed by the heat transfer surface T1 of plate P2 and the heat transfer surface T2 of plate P3, the heat transfer space S2 is formed by the heat transfer surface T3 of plate P4 and the heat transfer surface T4 of plate P5, and the heat transfer space S3 is formed by the heat transfer surface T5 of plate P6 and the heat transfer surface T6 of plate P7. The periphery of the plates is sealed in a known manner, such as by brazing, welding, or soldering, and this provides a closed perimeter 102 for the heat transfer spaces. This perimeter 102 is indicated only diagrammatically in the drawings.

In conventional double-wall heat exchangers, the gap may be occupied by air. However, in accordance with a preferred embodiment of the present invention, the heat transfer spaces S1-S3 are filled by a thermally conductive medium M. As used herein, “thermally conductive medium” denotes a medium that is more conductive than air. Preferably, the thermal conductivity of the thermally conductive medium exceeds about 0.134 W/(m K) at 288K temperature. More preferably, the thermal conductivity of the thermally conductive medium exceeds about 0.182 W/(m K) at 288K temperature, and most preferably, the thermal conductivity of the thermally conductive medium is at least about 0.200 W/(m K) at 288K temperature. Suitable heat transfer media include, without limitation, water, propylene glycol, ethylene glycol, HVAC&R refrigerants, nano-fluids containing aluminum oxide, copper oxide, or titanium oxide.

The thermally conductive medium M may be a gas, a liquid, a solid such as a wire mesh or porous foam, a gel, a slurry, a suspension, a colloidal dispersion, or a phase change medium. The medium M may be a single phase medium, such as a water. Alternately, the medium M may be a phase change medium, such as a refrigerant. In one embodiment of the invention, the medium M comprises a nano-fluid in which the nano-particles include metal particles of any size, form and shape having a thermal conductivity greater than about 6.6 W/(m K) at 288K temperature. More preferably, the thermal conductivity of the nanoparticles exceeds about 13.2 W/(m K) at 288K temperature, and, most preferably, the thermal conductivity of the nanoparticles is at least about 19.8 W/(m K) at 288K temperature.

Referring still to FIG. 1, conductivity of the heat transfer spaces S1-S4 may be enhanced by introducing at least one extra and preferably a plurality solid heat transfer contact points, designated generally as “CP,” that provide direct surface-to-surface heat transfer connections between the heat transfer surfaces of the first conduit and the heat transfer surfaces of the second conduit. The size, number, and position of these contacts CP may vary. These contacts may be made of metal and most preferably will be formed of a relatively heat conductive metal. Such metals may include copper, aluminum, titanium, and stainless steel and may take the form of a solid or a porous surface. These contacts CP may be formed by any suitable process. For example, metal beads may be positioned in the space S to provide separate metal-to-metal contact to improve heat transfer between the hot and cold heat exchange fluids. Alternately, the contacts CP may be formed by soldering, brazing, welding or a combination of these techniques. Still further, the heat transfer contacts CP may be used alone, that is, without the thermally conductive medium M, or in conjunction with the medium.

In the event of a failure of one of the plates that results in heat exchange fluid leaking into the a heat transfer space, the heat exchanger 100 may include at least one rupture point “R” in the perimeter for each of the heat transfer spaces S1-S3. These rupture points R will be designed to burst and permit release of thermally conductive medium M if the internal pressure of the heat transfer space exceeds a predetermined level.

Turning now to FIG. 2, various features of the present invention will be explained as applied to a tube-in-tube heat exchanger designated generally at 200. In this type of heat exchanger (also called concentric tube or double-pipe type heat exchangers), opposing flow paths are formed by concentric tubes. Thus, in the exemplary form shown, the heat exchanger 200 comprises an innermost tube 202, an outermost tube 204, and an intermediate tube 206. The innermost tube 202 forms a flow passage A for a first heat exchange fluid to flow in a first direction (downward as viewed in the drawings). The annulus formed by the outer diameter of the tube 206 and the inner diameter of the tube 204 forms a flow passage B that directs a second heat exchange fluid in the opposite direction (upward as viewed in the drawing). Thus, the flow passage A comprises the first conduit for a first heat exchange fluid, and the flow passage B comprises the second conduit for the second heat exchange fluid. It will be understood that in some embodiments not depicted in the drawings, the flow in the passages A and B may be parallel or in the same direction, rather than in opposite directions.

With continued reference to FIG. 2, the heat transfer space S is the annular space between the outer diameter of the inmost tube 202 and inner diameter of the intermediate tube 206. Thus, the heat transfer space S is formed by the heat transfer surface T1 of tube 206 and the heat transfer surface T2 of the tube 202. The periphery of the tubes, that is, the ends of the tubes 202, 204, and 206, are sealed in a known manner, such as by soldering, brazing, welding, or a combination of these techniques, and this provides a closed perimeter 208 for the heat transfer space S. This perimeter 208 is indicated only diagrammatically in the drawings.

As in the previous embodiment, thermally conductive medium M preferably occupies the heat transfer space S, and the heat exchanger 200 may include at least one rupture point “R” in the perimeter 208 for allowing release of the medium in the event of failure of one of the tubes. Additionally, a plurality of solid heat transfer contacts CP may be included to provide direct surface-to-surface heat transfer connections as previously explained.

Now it will be appreciated that the use of the heat transfer medium as a filler in the heat transfer space as well as the placement of heat transfer contact points CP increase the effectiveness of the heat exchanger and provide improved system performance. The improved performance permits the size of the heat exchanger to be reduced which in turns allows the size of the overall footprint of the entire HVAC&R system to be reduced.

The embodiments shown and described above are exemplary. Many details are often found in the art and, therefore, many such details are neither shown nor described herein. It is not claimed that all of the details, parts, elements, or steps described and shown were invented herein. Even though numerous characteristics and advantages of the present invention have been described in the drawings and accompanying text, the description is illustrative only. Changes may be made in the details, especially in matters of shape, size, materials, and arrangement of the parts within the principles of the invention to the full extent indicated by the broad meaning of the terms of the attached claims. The description and drawings of the specific embodiments herein do not point out what an infringement of this patent would be, but rather provide an example of how to use and make the invention. Likewise, the abstract is neither intended to define the invention, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. Rather, the limits of the invention and the bounds of the patent protection are measured by and defined in the following claims. 

What is claimed is:
 1. A double-wall heat exchanger comprising: a first conduit for a first heat exchange fluid and having a heat transfer surface; a second conduit for a second heat exchange fluid and having a heat transfer surface; wherein the heat transfer surface of the first conduit and the heat transfer surface of the second conduit together define a heat transfer space bounded by a perimeter; and wherein the heat transfer space is occupied by a thermally conductive medium.
 2. The double-wall heat exchanger of claim 1 wherein the heat exchanger is a plate heat exchanger.
 3. The double-wall heat exchanger of claim 1 wherein the heat exchanger is a tube-in-tube heat exchanger.
 4. The double-wall heat exchanger of claim 1 wherein the perimeter of the heat transfer space includes a rupture point configured to permit release of thermally conductive medium if the internal pressure of the heat transfer space exceeds a predetermined level.
 5. The double-wall heat exchanger of claim 4 wherein the heat exchanger comprises a plurality of solid heat transfer contacts providing surface-to-surface heat transfer connections between the heat transfer surface of the first conduit and the heat transfer surface of the second conduit.
 6. The double-wall heat exchanger of claim 1 wherein the thermal conductivity of the thermally conductive medium exceeds 0.134 W/(m K) at 288K temperature.
 7. The double-wall heat exchanger of claim 1 wherein the heat exchanger comprises a plurality of solid heat transfer contacts providing surface-to-surface heat transfer connections between the heat transfer surface of the first conduit and the heat transfer surface of the second conduit.
 8. The double-wall heat exchanger of claim 7 wherein the solid heat transfer contacts are formed of wire mesh or foam filling the heat transfer space.
 9. The double-wall heat exchanger of claim 1 wherein the thermally conductive medium is a single phase medium.
 10. The double-wall heat exchanger of claim 1 wherein the thermally conductive medium is a phase change medium.
 11. The double-wall heat exchanger of claim 1 wherein the thermally conductive medium comprises a nano-fluid.
 12. The double-wall heat exchanger of claim 11 wherein the nanoparticles in the nano-fluid include metal particles having a thermal conductivity greater than 6.6 W/(m K) at 288K temperature. 